Before today (17/08/2019), you can only request for email in the last question i.e the third question.
We just released Group Leads version 1.1.8 which allows you to request for email to be added to your email marketing software from any part of your Facebook group request questions.
Maybe you want to do A/B testing to see which one performs better than the other, now you can do it.
You can ask for email in the first or second or third (default) question and then add new members emails to your email marketing software when you approve new members.
Here is a quick video on how to set it up.